1. Field of the Invention
This invention relates generally to the subject matter of inorganic and biological CaCO.sub.3 formation. More particularly, it relates to the inhibition of CaCO.sub.3 -containing deposits by poly amino acid derivatives. These derivatives have been found effective for the inhibition of the formation of inorganic or biological CaCO.sub.3 -containing deposits on a surface with which they are contacted.
2. Description of the Prior Art
Control of CaCO.sub.3 -encrustrations and growth of calcifying organisms on surfaces in marine environments has long been recognized as a potentially solvable problem. By preventing or slowing the occurrence of these "fouling" substances in organisms, the useful lifetime of surfaces such as hulls of ships and pilings of docks can be increased. In the case of hulls of ships, prevention of fouling also has the effect of allowing the ship to move more efficiently through the water.
Historically, the problem has been approached by impregnating or coating surface with compounds that interfere with the metabolism of fouling organisms. For example, the use of inhibitors of carbonic anhydrase, an enzyme often involved in calcification, has been suggested for such use (Costlow, J. D., Physiological Zoology, 32:177 (1959)). More recently, inhibitors of the enzyme polyphenol oxidase, also involved in the calcification process have been shown effective as anti-fouling compounds (Turner, R. D., Symposium on Marine Biodeterioration, Naval Institute Press, Washington, D.C.). Less specific metabolic inhibitors, such as organotin compounds, are also being applied (Good, M. L., Symposium on Marine Biodeterioration, supra).
ln addition, CaCO.sub.3 crystal growth occurs abiotically in most natural solutions leading to unwanted calcified deposits. For example, scale builds up anywhere in the sea where nucleation occurs, because sea water is supersaturated with respect to CaCO.sub.3 by a factor of 5 to 10-fold, allowing crystal growth to proceed spontaneously (Stumm, W. and Morgan, J. J., Aquatic Chemistry, John Wiley and Sons, Somerset, N.J. (1981)). Inorganic scales are also often encountered as unwanted deposits in pipes and boilers where supersaturation becomes a problem due to evaporative concentrations of ions. Carboxylates, such as NTA, ethylene diamine tetraacetate (EDTA) and gluconates have been used to retard or inhibit the precipitation of supersaturated solutions of calcium carbonate, alrhough somewhat high concentrations are needed for these compounds to act as effective inhibitors. Hexametaphosphate at 1-10 ppm concentration was found to retard scaling, leading to the widespread use of polyphosphates as scale inhibitors in municipal and industrial water systems. (Monsanto's Technical Bulletin No. IC/SCS-323, Dequest 2010 Phosphonate).
In recirculating cooling water systems, calcium carbonate is generally the predominant scalant. Since cooling towers are efficient air scrubbers, this circulating water is saturated with CO.sub.2, establishing an equilibrium between bicarbonate and carbonate in solution. As the pH of the water rises, this equilibrium shifts towards carbonate. Heating also produces a shift in the dissolved inorganic carbon equilibrium to the right, producing calcium carbonate: ##STR1## Finally, calcium carbonate shows an inverse solubility trend, being less soluble at higher temperatures. All of these factors tend to produce scaling on critical heat-transfer surfaces which reduces the heat transfer efficiency, increases frequency of required cleaning and decreases the life of the system. Several of the inhibitors of the precipitation of calciuum carbonate show the phonomenon of a threshold effect, i.e., the prevention of precipitation from supersaturated solutions of scalants by substiochiometric levels of inhibitors. Present mechanistic theories postulate that the threshold agent is absorbed on the growth sites of the scalant crystallite during the process of crystallization and alters the growth pattern so that the resultant scalant crystals are formed more slowly and are highly distorted. (Reddy M. M. and Nancollas, G. H., Desalination 12:61 (1973)).
A speculative model of organic matrix structure and function, based primarily on aspects of mollusk shell proteinaceous matrix biochemistry, as well as a brief review of the proteinaceous organic matrices from various other phyla was presented by Weiner, S., Traub, W. and Lowenstam, H. A., "Organic Matrix in Calcified Exoskeltons", in Biomineralization and Biological Metal Accum., pp. 205-224 (1983), Westbroek and De Jong, Eds., Reidel Publishing Co. Further characterization of the various matrical components, such as the soluble matrical fraction containing glycoprotein components can be found in Krampirz, G., Drolshagen, H., Hausle, J., and Hof-Irmscher, K, "Organic Matrices of Mollusk Shell", in Biomineral. and Biol. Metal Accum. supra, pp. 231-247 (1983), incorporated herein by reference. Calcium-binding, sulfated, high molecular weight glycoproteins have been identified in the soluble matrix of several species. In addition, this soluble fraction may also contain a number of smaller molecular weight glycoprotein components (Weiner, F. Lowenstam, H. A. and Hood, L. J., J. Exp. Mar. Biol. Ecol., 30:45-51 (1977), incorporated herein by reference). A further characterization of the amino acid sequence of soluble mollusk shell protein by peptide analysis after cleavage of the proteins on both sides of the Asp residues, showed a pattern of a repeating sequence of aspartic acids separated by either glycine or serine in an alternative manner with Asp. The repeating sequence observed is of the form (Asp-Y).sub.n -type, where Y is a single amino acid. The natural organic matrix of almost all mineralized tissues studied to dare (both vertebrates and invertebrates) contain proteins which are enriched in aspartic acid (Asp) and/or gluramic acid (Glu) (Veis, A. and Perry A., Biochemistry 6:2049 (1967); Shuttleworth, A. and Yeis, A., Biochem. Biophys. Acra, 257:414 (1972)).
The (Asp-Y).sub.n -type sequence was hypothesized to be present in the organic matrices from a variety of molluscan species, such as Crassostrea virginicia, Mercenaria mercenaria, Crassostrea irredescens and Nautilus pompilius, and suggested that these sequences played a function as a template for mineralization, although X-ray diffraction studies showed that there was a poor match between the Ca - Ca distances in the crystal lattice and the potential calcium-binding sites along the polypetide chain for this sequence (Weiner S., and Hood L., Science 19: 987 (1975); Weiner S., in The Chem. and Biol. of Mineral. Connective Tissues, Veis A., ed., pp., 517-521, Elsevier North Holland, Inc. (1981); and Weiner S. and Traub W., in Struct. Asp. of Recog. and Assembly in Biol. Mascromolec. Balaban, N., Sussman, J. L., Traub, W. and Yonath, A., Eds., pp. 467-482 (1981) incorporated herein by reference).
Acknowledging that the process of CaCO.sub.3 nucleation and crystal growth itself is central to the process of encrustration by all calcifying organisms, such as barnacles, oysters, ship worms, algae and the like, Wheeler, A. P., George, J. W. and Evans, C. A., Science 212: 1397 (1981), incorporated herein by reference, made the discovery that a 170,000 MW glycoprotein obtained from the proteinaceous matrix that permeates the CaCO.sub.3 of oyster shell is a very potent inhibitor, rather than an initiator of CaCO.sub.3 nucleation and crystal growth as previously throught. The 170,000 glycoprotein was identified by staining for carbohydrates and it was shown to contain 10.2% carbohydrate by weight. The molecular weight and carbohydrare content reported for the glycoprotein form oyster shell are comparable to those observed for the protein obtained from clams by Crenshaw, M. A., Biomineralization 6: 6 (1972), incorporated herein by reference.
Wheeler, A. P., and Sikes, C. S., in concurrently filed and copending application entitled "Inhibition of the Formation of Inorganic or Biological CaCO.sub.3 -Containing Deposits by a Proteinaceous Fraction Obtained From CaCO.sub.3 -Forming Organisms", incorporated herein by reference, disclose a method of inhibiting the formation of CaCO.sub.3 -containing deposits with a glycoprotein-containing fraction isolated from CaCO.sub.3 -containing tissues obtained from CaCO.sub.3 -forming plants or animals. As such, the glycopeptide-like materials have been shown to have a broad range of MW ranging from 400 to 10.sup.8, and higher.
Sikes, C. S., and Wheeler, A. P., in concurrently filed and copending application entitled "Inhibition of Inorganic and Biological CaCO.sub.3 Deposition by a Polysaccharide Fraction Obtained from CaCO.sub.3 -forming Organisms" incorporated herein by reference, disclose a method of inhibiting the formation of CaCO.sub.3 -containing deposits by applying a polysaccharide-containing fraction, substantially free of protein components, isolated from CaCO.sub.3 -containing tissues obtained from CaCO.sub.3 -forming plants or animals. As such, the polysaccharide-containing materials have been shown to have a broad range of MW ranging from 500 to 10.sup.8, and higher.
Sikes, C. S., and Wheeler, A. P., in concurrently filed and copending application entitled "Inhibition of Inorganic or Biological CaCO.sub.3 Deposition by Synthetic Polysaccharide Derivatives", incorporated herein by reference, further disclose a method of inhibiting the formation of inorganic or biological deposition of CaCO.sub.3 by applying to a surface in contact with CaCO.sub.3 a synthetic saccharide polymer having a polysaccharide-matrix-like structure.
None of the cofiled, copending applications by the present inventors are considered prior art to the present invention.
However, prior to the present invention a method of preventing or inhibiting the formation of CaCO.sub.3 -containing deposits using an artificial polypeptide material of ready availability which operates in a specific, nontoxic manner was not known. Development of such a method will greatly aid in reducing the harmful effects of CaCO.sub.3 -containing deposits, such as those discussed above.